The Stuff of Nightmares
by Imbrus-the-Wolf
Summary: Detective Kate Beckett has always been a skeptic. She's never believed in ghosts, or anything paranormal. But when she and her open-minded partner Richard Castle stumble into the world of the Winchester brothers and their friend Castiel, who hunt creatures of that persuasion for a living, she is forced to re-evaluate everything she believes, and catch a new kind of killer!
1. Zombie

**A/N: Hello! This is not the first fan-fiction I've written, but it is the first fan-fiction I've published.  
**

**I'm not 100% sure if I'll ever finish it, but I do hope to try.**

**The cross-over was inspired by user kevin_fricking_solo on Instagram. This person pointed out that Mark Pellegrino was in an episode of Castle, and I sort of had to. However, I don't think Lucifer will be making an appearance, unfortunately.**

**I didn't include a secondary genre because I'm just going to see where this takes me; I have no super earth-shattering plot planned. So there should probably be a bit of everything: some humor, some fluff, some angst, a little drama, and a bit of romance.**

**Rated T for safety.**

**This is set sometime before Beckett and Castle started dating; I'm not really set on when exactly. As for Supernatural, I think it's going to be completely AU; no big boss battles that occur in the show will be part of this fic. However, adventures that Team Free Will have had in the past may or may not be referenced.**

**Main characters: Kate Beckett, Richard Castle, Sam Winchester, Dean Winchester, and Castiel. Perhaps more to come. There might be a little bit of Destiel. Maybe a lot. Not sure yet. Third person point of view, and right now it's from Beckett's perspective, though it might be someone else's depending on the chapter.**

**Disclaimer: I don't own Castle or Supernatural, or the characters in it. If I owned my very own Sam Winchester and/or Castiel, you think I'd be _here_?**

**Sorry that this note was so long, and that it's also kind of... unspecific. A summary of that whole note: I don't know what I'm doing.**

**Ahem. Anyway... Read if you wish to, and enjoy... whatever this is.**

Detective Katherine Beckett stepped through the splintered door-frame, ducking underneath the crushed beams. She weaved around smashed furniture, broken glass crunching under her feet. Through the destroyed foyer and into the living room, where Lanie was crouched next to a body. Beckett cocked her head, noticing that underneath the bent corner of a hand-woven Peruvian rug was the edge of something written in spray-paint. She made a mental note of it before walking over to where Lanie sat on her haunches.

"What've we got, Lanie?" Beckett asked, threading a hand through her wavy, chocolate-brown hair. She shifted her weight to her right foot, her fingers curled around a paper cup of coffee.

"The vic was female. She was in her late twenties. Jane Doe." Lanie looked up at Beckett, clearly waiting for the detective's obvious next question.

"Cause of death?"

"Gunshot to the head. But here's the weird bit." Lanie lifted the woman's hands to show Beckett. The blonde's fingernails were chipped, and her fingertips were soaked in blood. Beckett's brow furrowed as Lanie continued. "There's a gaping wound in the roof of her mouth that goes right up to intersect with the bullet hole. See that?" Lanie pointed to something drenched in blood on the floor not half a foot away. She picked it up and let Beckett see. "It's a bullet. The bullet that killed her."

"She dug the bullet out of her own brain and _then_ died?" Beckett muttered. "That's not possible."

"Yeah. She was shot in the head. She'd have been dead before she hit the ground, and yet somehow, she pulled this," Lanie answered, brandishing the tiny bullet at Beckett for emphasis, "out of her head and bled out."

"Wow," said a voice behind Beckett. The detective didn't have to turn around to know who it was: the ruggedly handsome author who'd been tailing her, Richard Castle. "You know what this is, right?"

Beckett sighed exasperatedly. "Alright, get it out. What?"

"She's a zombie, _duh_!"

"I thought zombies died when you shot them in the head," Beckett pointed out.

"I wasn't finished yet. You interrupted me. No, this isn't your average, run-of-the-mill, _The Walking Dead_ zombie. This is a special breed of zombie that, when shot in the head, can survive a few seconds afterwards."

"What, like a _next-generation_ zombie?" Beckett scoffed, folding her arms. "Kind of a small step, don't you think?" She neglected to mention that it probably would've taken several minutes, as opposed to seconds, for the Jane Doe to dig a hole straight through her own head. Not to mention, having the bullet there probably would've hurt a lot less than tearing away flesh to pull it out.

"Evolution takes a long time, Beckett; complete immortality won't just happen overnight. But pretty soon – probably within a few years – zombies won't die _at all_."

"Right," Beckett said, the single syllable dripping with sarcasm, and turned back to Lanie, who was examining the bullet with furrowed brow. Beckett tilted her head and squinted at the tiny red-stained piece of metal. "What is it?"

"Look at this," Lanie said, and held the bullet up to the light once more. Beckett leaned forward to inspect it closely. Engraved on the front was a tiny symbol – a five-pointed star within a circle. The vertex of each angle touched the inner edge of the circle, connecting the two shapes.

"A pentagram," she mused aloud, standing up straight again. "In some cultures, it's a spiritual symbol. The four bottom points represent air, water, fire, and earth, and the top one represents the soul."

"And in some cultures," Castle said, mimicking Beckett's tone, "it's _Satanic_. Satanists _and_ zombies! This is going to be awesome."

"Anything else?" Beckett asked Lanie, ignoring Castle's remark.

"Yeah. One more thing." The medical examiner reached into the woman's pants pocket.

"Whoa, Lanie, don't you want to buy her dinner first?" Castle quipped, and Lanie rolled her eyes before pulling her hand out of the girl's pocket.

Coated on her fingers was a thick layer of yellow dust. Beckett crinkled her nose as the smell of rotten eggs wafted upwards.

"Sulfur. And lots of it." The M.E. indicated around the room, and Beckett spotted little clumps of the powdery stuff on window sills and scattered on the floor.

"Huh," Beckett muttered.

Castle, who had moved over to where the Peruvian rug lay rumpled on the floor, began coughing violently into his fist. Once the attack had ended, he said, loudly and clearly, "_Brimstone_."

Beckett just rolled her eyes.

She turned to Lanie and opened her mouth to speak, when Castle interrupted her.

"_Kate_," he said, and his tone, which was suddenly serious and urgent, prompted Beckett to turn and face him. He was crouched down, staring intently at a section of the ground that was out of Beckett's line of view, blocked by an armchair. She could see that he had moved the rug away, and his expression was one of shock. It morphed suddenly into a childlike glee. "Kate," he said again, grinning widely. "Come see this."

Beckett stepped around the big chair to stand by Castle, followed by Lanie's eyes. She arched her eyebrows and parted her lips in surprise when she saw the object of Castle's focus.

Spray-painted onto the hardwood floor was a massive red pentagram, but this one was adorned with several archaic symbols. There was one disturbance in the otherwise perfect circle; a tiny section of the paint was scratched out, breaking the pattern.

Castle grinned at her. "Believing the Satanist theory _now_?"


	2. Case

**A/N: Thank you so much to all those who read, favorited, followed, and/or reviewed my story so far! It means a lot to me, and I was surprised by the number of people who liked it; I expected one or two follows to trickle in every few days. So – thank you.**

**Here's the next chapter.**

"Hey, man, look at this," Sam said, turning his laptop around so that Dean could see. His brother, who lounged on the other side of the table as he waited for his breakfast, turned his attention away from the busty waitress wiping down the neighboring table and squinted at Sam's screen. "A woman was shot dead in her own home. The house was in ruins when the cops found her."

"So? That sounds like a run-of-the-mill murder more than a case, if you ask me," Dean said, skimming the report. The headline read **WOMAN SHOT IN OWN APARTMENT – SATANIC SYMBOLS DECORATE LIVING ROOM**. "Well, I guess that's a little bit out of the ordinary," he allowed, pointing to the headline. "What symbols?"

Sam pursed his lips, scrolling down a little bit. Interrupting the deadpan black text was an image. Dean arched his eyebrows. It was a devil's trap, drawn in red spray paint.

"Alright," Dean said, leaning back in his seat. "So, the girl was a hunter? Maybe killed by something that had a beef with her? What monster do we know of that shoots people?"

"I don't think so." Sam shook his head and pointed to the image. "Look closer." He clicked the photo and it expanded, filling the screen.

"It's broken," Dean said with dawning comprehension, staring intently at the portion of the outer circle that was scratched out.

"Which means that our vic – Sofia Townes – was a demon." Sam closed the image. "And get this," he said, highlighting a section of the text. "She was shot in the head. But when they examined the body, they found a hole in the roof of her mouth, right up to where the bullet went in. There was blood on her fingers, which was confirmed to be her own."

"So… What?"

"Well, it looks like she carved that hole through her own head."

"…Okay, so… Sofia lived in that apartment, and one day a demon pops in. A hunter comes by while Sofia's out, draws a devil's trap on the floor, and waits for Sofia to get home. She does, and the hunter… what, shoots her? If he knew enough to draw the trap, shouldn't he know that they don't die when shot?"

Sam furrowed his brow and scrolled down a bit more. "I don't know. Maybe I missed… Oh." He stopped at an image of a bullet with a devil's trap carved into it.

"Okay, I've got it. So the hunter shoots the demon so it can't smoke out. So he wants information or something. He leaves the demon there, planning to come back later. While he's gone, the demon digs the bullet out. The trap gets broken and the demon leaves Sofia, who crumples."

"Okay, but how did the trap get broken?"

"And what did the hunter want to know?"

Both Winchesters were silent as the waitress delivered Dean's stack of pancakes and bacon.

"Have a nice day, boys," she said with a thick southern accent and a big smile.

Dean nodded in thanks and began pouring syrup over his pancakes. "So," he said around a mouthful of food. "When was this?"

"Four days ago, in New York City."

Dean stopped chewing. "The Big Apple?" He grinned. "Awesome."

Sam lifted one eyebrow and stared at Dean.

"What?" Dean said, swallowing his breakfast. "I've never been."

"Really?" Sam asked, genuinely surprised.

Dean shrugged and took another bite of his pancakes.

"Well, we're in Lansing, Michigan. If we leave now, we should be able to get to New York by nine o'clock tonight."

Dean blinked at Sam. "Now? Like, _now_-now?"

Sam stared at Dean for a moment. "Yeah," he said shortly. "Now-now."

Sam collected his things and walked towards the door of the diner. Dean looked forlornly down at his unfinished pancakes before pulling a crumpled handful of cash out of his pocket and putting it on the table.

**A/N: So, this one is even shorter than the last. Sorry about that. Hope you enjoyed! I have absolutely nothing to do today, so I may be able to add Chapter 3 sooner rather than later.**


	3. Agents

**A/N: Thank you to those who read and enjoyed the last chapter!**

The elevator doors slid open and Castle sauntered out. He trotted eagerly over to where Beckett leaned against her desk and glared at the murder board like it had personally victimized her.

"Hey," he said by way of greeting, and handed her another hot cup of coffee. "I got more coffee. How're things shaping up?"

"They're not," Beckett groused, accepting the coffee. She furrowed her brow. "CSI only got one set of fingerprints off the scene." She pointed a slender finger at a grainy image on the board. "Jeremy Bergeron, aged 46."

"Well, did you interrogate him?" Castle asked.

"Can't, Captain of the Obvious," Beckett said. "He's been dead for four years."

Castle was silent. His features were frozen, eyes wide and mouth open. Then his lips curled upward into a grin. Reverently, he whispered, "_Zombie_."

"Castle, would you drop the whole zombie thing? There's no way that our perp was anything but human," Beckett said impatiently.

"You're right," said a gruff voice from behind Castle. "Absolutely no way. 100% monster-free zone."

Beckett and Castle both turned their attention from the murder board to the two men standing by in black suits. One was tall, with long brown hair and soft brown eyes. He gave a small nod in favor of a "hello". The other was Castle's height, with messy dirty-blond hair and intense green eyes. He walked with a sort of confidence that the taller man lacked.

In a smooth motion that wasn't so much well-rehearsed as it was oft-used, both men pulled out badges. Then, in unison again, they tucked the badges into their suit pockets.

"I'm Agent Krueger; this is my partner, Agent Voorhees," said the green-eyed agent. "We're with the FBI. We're here investigating the recent murder."

Beckett nodded. "I'm Detective Beckett. This is Castle. We—"

"Wait, wait, wait," Castle interrupted her. "Let me see those badges again."

The agents shared a glance, then held out the requested articles. Castle took Krueger's badge and examined it closely, his lips pursed. Then he gave a curt nod and handed the badge back. He shared a glance with Beckett, who seemed slightly irritated. Whether it was about his interruption, or the dead-end case, Castle wasn't sure.

"_As I was saying_," Beckett continued, "we've found no evidence that anyone living could have committed this crime. The only fingerprints—"

"Yeah, yeah, we heard you. Bergeron – stone cold," Krueger said, waving his arms.

"Well," Beckett said through gritted teeth, "do _you_ have any ideas?" Castle stifled a laugh at how much this man was pissing her off.

The two agents shared a glance.

"Nope. Nothing," said Krueger.

"But if we do think of anything relevant, you'll be the first to know," Voorhees said, speaking up for the first time.

"Well, t—that's it? You're leaving now?" Castle asked as the two agents turned away.

"Um… Yeah," Krueger said, giving a short nod. "Adios." He mock saluted, and began sauntering away. As he headed towards the elevator, Agent Voorhees at his heels, Krueger turned to his partner and said, "Okay, time to go eat. I haven't had anything since breakfast."

"That was odd. They heard one little piece of info, then didn't need anything at all. FBI agents are usually a lot more… thorough about these things," Beckett whispered.

"Well, it's probably because they're not real FBI agents," Castle muttered back.

"What?"

"I looked at their badges, remember? They were really good fakes, but fakes nonetheless. Those guys aren't the real Mulder and Scully. They're playing dress-up." He grinned gleefully. "This case just keeps getting better."

"How did you know to check their badges?"

Castle arched an eyebrow quizzically, as though it were obvious. "Uh," he said, "_Krueger_ and _Voorhees_? Those are classic movie villains."

Beckett furrowed her brow in concentration. "You're right. _Nightmare on Elm Street_ and _Friday the 13__th_. I don't know how I didn't get that before. I guess I just wasn't paying attention. I was too caught up in the case."

"Yeah, well," Castle said, "how about we find some time to bond with Freddy and Jason?" He looked up at where the elevator doors had just closed, carting the two imposters away. "How about a nice little dinner invite to get closer to them?"

"Castle, that's a terrible idea. They'll just figure we know something's up. For now we have to lay low and – Castle!"

The writer was already bounding towards the steps. Beckett raced after him, but he had a good enough head start that by the time she caught up with him, they were out on the street, chasing down a black muscle car. Inside it, Beckett could see the two fake agents arguing about something.

Castle dashed alongside the car as it rolled at a leisurely pace down the street. He banged on the side with on open palm, earning a startled jump from Agent Krueger.

Beckett and Castle stood side-by-side next to the car – a '67 Chevy Impala – as Krueger rolled down the window.

"What?" he said gruffly.

"I wanted to know," Castle said between deep breaths, "if you guys might want to have dinner together." The phony agent arched an eyebrow, and Castle clarified, "The four of us. To discuss the case."

Krueger and Voorhees shared a glance. Voorhees glared, and shook his head almost imperceptibly. Krueger looked somewhat disappointed, and started to say, "Sorry, but—"

"I make a mean apple pie," Castle said in a sing-song voice, and Krueger's face brightened.

"Come on," he whispered to Voorhees. "It couldn't hurt. And I'm sure they have some helpful insight on the case." He rolled the window up so he and his partner could talk in private. Beckett focused on reading their lips, shushing Castle when he tried to speak.

Voorhees sighed. "We don't _know_ them, man. Besides, they're cops. It's a bad idea."

If Beckett's little red flag wasn't already high in the air, that sure did the trick. _They're cops_.

"But _pie_." Krueger looked at Voorhees pleadingly, and finally the taller man gave in.

"Fine," he said. "But I still think it's not a good idea."

Krueger rolled his window down and said, "Alright. But we'll only stay a little while. We've got stuff to do."

Castle grinned and pulled a pen out of his back pocket. He scribbled down his address and handed it to the two men. "Meet you there in ten minutes?"

"Probably more like thirty. We've got some stuff to do first." The Impala then turned away and sped down the street.

"Castle," Beckett said as they turned back toward the precinct, "that was a terrible idea. You can't just invite federal agents to _dinner_. Normal people don't do that!"

"Well, A, they're not federal agents. B, they won't know that that's out of the ordinary, because – guess why – they're _not federal agents_. C, it was an awesome idea."

Beckett shook her head and sighed resignedly. "You clearly didn't think it through at all," she muttered.

Castle, pretending not to hear her, said, "You can ride with me in my car."

**A/N: Hope you enjoyed it! Despite my lack of a master plan, there is a reason behind everything I do.**

**My reason for this mini dinner party is for shameless fluff and maybe a shocking revelation from one of the dynamic duos attending.**

**Sorry I took so long to update, but I've been busy! I wish I could spend all my time writing fan-fic, but I've got other obligations, unfortunately.**

**I do look forward to seeing y'all when I put the next chapter up, which will hopefully be in a much shorter amount of time than this one took to write.**


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